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Inc. 4 Intended status: Informational May 9, 2008 5 Expires: November 10, 2008 7 The stale-while-revalidate HTTP Cache-Control Extension 8 draft-nottingham-http-stale-while-revalidate-01 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on November 10, 2008. 35 Abstract 37 The stale-while-revalidate HTTP response Cache-Control extension 38 allows servers to instruct caches to serve stale responses while 39 validating them, to avoid latency in some situations. 41 Table of Contents 43 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 44 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 45 3. The stale-while-revalidate Cache-Control Extension . . . . . . 3 46 4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 47 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 48 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4 49 7. Normative References . . . . . . . . . . . . . . . . . . . . . 4 50 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5 51 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 52 Intellectual Property and Copyright Statements . . . . . . . . . . 6 54 1. Introduction 56 The potential for latency (due to the network as well as server 57 processing) introduced by cache validation in HTTP [RFC2616] is often 58 undesirable; while subsequent requests can be served from the cache 59 quickly, the request that triggers validation sees degraded service. 61 In some situations, it may be useful to avoid this latency, at the 62 cost of serving slightly stale responses. the stale-while-revalidate 63 HTTP response Cache-Control extension allows caches to do this. 65 2. Notational Conventions 67 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 68 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 69 document are to be interpreted as described in RFC 2119 [RFC2119]. 71 This specification uses the augmented Backus-Naur Form of RFC2616 72 [RFC2616], and includes the delta-seconds rule from that 73 specification. 75 3. The stale-while-revalidate Cache-Control Extension 77 When present in an HTTP response, the stale-while-revalidate Cache- 78 Control extension indicates that caches MAY serve the response it 79 appears in after it becomes stale, up to the indicated number of 80 seconds. 82 stale-while-revalidate = "stale-while-revalidate" "=" delta-seconds 84 If a cached response is served stale due to the presence of this 85 extension, the cache SHOULD attempt to revalidate it while still 86 serving stale responses (i.e., without blocking). 88 Note that 'stale' implies that the response will have a non-zero Age 89 header and a warning header, as per HTTP's requirements. 91 If delta-seconds passes without the cached entity being revalidated, 92 it MUST NOT continue to be served stale, absent other information. 94 4. Example 96 A response containing: 98 Cache-Control: max-age=600, stale-while-revalidate=30 100 indicates that it is fresh for 600 seconds, and it may continue to be 101 served stale for up to 30 seconds while an asynchronous validation is 102 attempted. If validation is inconclusive, or if there is not traffic 103 that triggers it, after 30 seconds the stale-while-revalidate 104 function will cease to operate, and the cached response will be 105 "truly" stale (i.e., the next request will block and be handled 106 normally). 108 Generally, servers will want to set the combination of max-age and 109 stale-while-revalidate to the longest total potential freshness 110 lifetime that they can tolerate. For example, with both set to 600, 111 the server must be able to tolerate the response being served from 112 cache for up to 20 minutes. 114 Since asynchronous validation will only happen if a request occurs 115 after the response has become stale, but before the end of the stale- 116 while-revalidate window, the size of that window and the likelihood 117 of a request during it determines how likely it is that all requests 118 will be served without delay. if the window is too small, or traffic 119 too sparse, some requests will fall outside of it, and block until 120 the server can validate the cached response. 122 5. Security Considerations 124 This document provides origin servers with a mechanism for dictating 125 that stale content should be served from caches under certain 126 circumstances, with the expectation that the cached response will be 127 revalidated in the background. It is suggested that such validation 128 be predicated upon an incoming request, to avoid the possibility of 129 an amplification attack (as can be seen in some other pre-fetching 130 and automatic refresh mechanisms). Cache implementers should keep 131 this in mind when deciding the circumstances under which they will 132 generate a request that is not directly initiated by a user or 133 client. 135 6. IANA Considerations 137 This document has no actions for IANA. 139 7. Normative References 141 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 142 Requirement Levels", BCP 14, RFC 2119, March 1997. 144 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 145 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 146 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 148 Appendix A. Acknowledgements 150 Thanks to Ben Drees, John Nienart, Henrik Nordstrom, Evan Torrie, and 151 Chris Westin for their suggestions. The author takes all 152 responsibility for errors and omissions. 154 Author's Address 156 Mark Nottingham 157 Yahoo! Inc. 159 Email: mnot@yahoo-inc.com 160 URI: http://www.mnot.net/ 162 Full Copyright Statement 164 Copyright (C) The IETF Trust (2008). 166 This document is subject to the rights, licenses and restrictions 167 contained in BCP 78, and except as set forth therein, the authors 168 retain all their rights. 170 This document and the information contained herein are provided on an 171 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 172 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 173 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 174 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 175 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 176 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 178 Intellectual Property 180 The IETF takes no position regarding the validity or scope of any 181 Intellectual Property Rights or other rights that might be claimed to 182 pertain to the implementation or use of the technology described in 183 this document or the extent to which any license under such rights 184 might or might not be available; nor does it represent that it has 185 made any independent effort to identify any such rights. 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